It is common practice to use a scintillator in combination with an image sensor to capture x-ray images. In such a setup, the image sensor is placed behind the scintillator. However, by laws of nature, the scintillator can only absorb a certain fraction of all x-ray photons that enter the scintillator's surface. As a consequence, a number of non-absorbed x-ray photons penetrate through the scintillator and continue onto the image sensor. Some of these x-ray photons are absorbed in the image sensor, which typically results in high noise affecting one or several pixels in the neighborhood of the location where the x-ray photon was absorbed.
It is a general requirement in x-ray imaging to achieve the best possible image quality, often interpreted as the balance between high resolution (sharpness) and low noise. These two image requirements are typically contradictory so that high resolution often is accompanied by high noise and vice versa. The noise generated by absorption of x-ray photons in the image sensor, as explained in the previous paragraph, cannot be easily filtered without degrading the resolution (sharpness) in the image.
Sometimes a fiber optic plate (FOP) made of x-ray-absorbing glass is placed between the scintillator and the image sensor in order to absorb x-ray photons that penetrate through the scintillator, thereby reducing noise. However, this solution is costly and increases the overall thickness and weight of the system, which is many times undesired.